When you adjust the tonal range in rgb, then as well as improving the light and darkness of the image, the colours will also change (as shown in the left image), but if you change the blend mode to luminosity, then only the tones will change not the colour (right image).
Perhaps you would be happy with the left adjustment - but you have lost control of the colours (which you should ideally adjust later in the work flow). The Action creates a second adjustment layer to allow you to alter the colours independently of the tones.
So this Action allows you to make changes in rgb and not loose control !
The Action creates you two Adjustment layers, one for Colour and the other for Luminosity.
In the Colour adjustment, change the master rgb curve to alter the Saturation (try an S-curve), and the individual channels to change the Colour.
In the Luminosity adjustment, you just need to change the master rgb curve and is the place to set your Black and White points using the Droppers.
This separation of tone and colour gives you much more control in rgb - Lab, of course, doesn't have this problem. I normally change the Luminosity first before thinking about Colour.